gulps I'll say it... Backstreet Boys! Just musically speaking I'll straight up say Iron Maiden, but holistically and also considering how you absolutely want only one answer, I'll have to go with BSB. I think it's because their songs are basically soundtracks to most of my fun memories growing up. Whenever I hear their songs it takes me back to those days of growing up, all that fun times with family and friends, all those girls I had crush on, all those innocent days.
Back during their prime, a cousin of mine had to stay in our place for like a year. She used to be play BSB everyday, so that's basically how I got introduced to them. I think it was around when I was in middle school. Started getting in Iron Maiden and the whole metal and rock stuff during high school, also when I started to play the guitar.